A flexible printed circuit board (hereinafter, referred to as ‘FPC’) has a printed circuit, which is provided on at least one surface of a flexible insulating film such as a polyimide film or a polyester film with or without an adhesive between the printed circuit and the flexible film. According to need, a flexible insulating film having openings is adhered on the surface of the printed circuit by an adhesive, or a protective layer is formed on the surface of the printed circuit. The openings are formed on the flexible insulating film so as to correspond to portions on which terminals used to mount circuit components or terminals used to be connected with an external board are formed. The protective layer is formed on the surface of the printed circuit by methods of coating, drying, exposing, developing, and heat-treating a photosensitive insulating resin. The FPC is widely used to build a circuit into a complex mechanism of an electronic device such as a mobile phone, video camera, personal lap top computer or the like which have been rapidly miniaturized and multi-functionalized. In addition, the FPC is also used to connect a moving unit such as a printer head with a control unit due to its excellent flexibility. A shielding countermeasure against electromagnetic wave has been required for the electronic device in which the FPC is widely used. Accordingly, a shielded flexible printed circuit board (hereinafter, referred to as a ‘shielded FPC’) having the shielding countermeasure against electromagnetic wave has been used as the FPC to be built in the device.
In addition, similar to a flex rigid board or a flexboard (registered trade mark), the following FPC has been used recently. In the FPC, the printed circuit board is partially laminated to form multilayered portions used to mount components, and cable portions extend outside from the multilayered portions. Specifically, Similar to the electronic device, shielding countermeasures against electromagnetic wave have been required for the cable portions of the boards.
The shielded FPC uses a flexible insulating film as a cover film. Furthermore, a shielding layer is provided on one surface of the cover film, and a releasable adhesive film having adhesion is adhered on the other surface of the cover film in order to form a reinforcement-shielding film. Then, the shielding film is adhered on at least one surface of the FPC by using a conductive adhesive and by heating and pressing the shielding film and the FPC. Further, after the shielding layer is electrically connected to the grounding circuit formed on the FPC by the conductive adhesive, the adhesive film is released. Here, when an electric connection between the shielding layer and the grounding circuit is not particularly required, a normal adhesive not having conductivity may be used instead of the conductive adhesive to adhere the shielding layer to the grounding circuit. The FPC is directly connected to a rigid circuit board, or the multilayered portions used to mount components are formed to be connected to the cable portions like the flex rigid board or the flexboard. In this case, according to need, a shielding layer is also provided on a rigid circuit board or the multilayered portions used to mount components similar to the above-mentioned structure. Therefore, it is possible to adequately manage electromagnetic wave.
However, there are many cases where the cover film used in the shielding film is made of engineering plastic such as polyphenylenesulfide (PPS), polyester, aromatic aramid, and the thickness of the cover film is thick (for example, 9 μm) or the stiffness thereof is strong. For this reason, the flexibility of the cover film deteriorates. Moreover, when the adhesive film is released from the shielded FPC and then a glass epoxy board or the like is adhered on the cover film to reinforce the shielded FPC, PPS barely adheres to the glass epoxy board or the like due to the fact that PPS has difficulty in adhesion.
In order to solve the above-mentioned problem, the following shielding film and a method of manufacturing the shielding film has been disclosed in JP-A-2004-95566. In the shielding film, a resin having excellent thermal resistance and adhesiveness is coated on one surface of a separation film via a release agent layer to form the shielding film, and an adhesive layer is further provided on the cover film via a metal layer. Here, since the cover film has excellent flexibility and is formed by the coating method, the thickness of the cover film is thin, that is, about 5 μm. In addition, since the cover film has adhesion, it is possible to easily adhere a glass epoxy board after the release of the separation film.